The present disclosure relates to a radiation imaging device and a radiation imaging display system which are suitable for X-ray radiography for medical care and non-destructive inspection, for example, and also to a transistor which is used in such a radiation imaging device.
In recent years, as a technique of acquiring images as electric signals (imaging techniques employing photoelectric conversion), techniques using a Charge Coupled Device (CCD) image sensor or a Complementary Metal Oxide Semiconductor (CMOS) image sensor have been mainstream. An imaging area in these image sensors is limited to the size of a crystal substrate (silicon wafer). However, in particular, in the medical field and the like in which imaging is performed using X-rays, an increase in the imaging area is desired and also, demand for moving-image performance is rising.
For example, as a chest X-ray machine for a human body, there is used the following radiation imaging device that obtains an image based on radiation as an electric signal without mediation of a radiographic film. That is a so-called radiation imaging device of indirect conversion type, in which a wavelength conversion layer is provided on a circuit board including a photoelectric conversion element such as a photodiode and a thin-film transistor (TFT). By such a configuration, radiation entering the device is converted into visible light in the wavelength conversion layer, and this visible light is received by the photoelectric conversion element. Reading of the photoelectric conversion element is carried out by the circuit including the TFT, and thereby an electric signal is obtained.
Further, in addition to the radiation imaging device of the indirect conversion type using the wavelength conversion layer as described above, there is also a so-called radiation imaging device of direct conversion type provided with a functional layer that directly converts radiation into an electric signal (for example, a direct conversion layer made of a-Se or Cd—Te). In this radiation imaging device of the direct conversion type, the radiation is allowed to enter the direct conversion layer and electric charge corresponding to the incident amount is accumulated in a capacitor provided in the circuit board, and the electric charge is read by the transistor, and thereby an electric signal based on the amount of the entering radiation is obtained. The transistor used in these radiation imaging devices (of the indirect conversion type, and the direct conversion type) has, for example, a gate insulator between a gate electrode and a semiconductor layer forming a channel, and the gate insulator is formed to include a silicon oxide film.
Here, in a case where the silicon oxide film is used (or in a case where a laminated film including silicon oxide is used) as the gate insulator of the transistor, when radiation is taken into such a gate insulator, electrons in the film are excited by a photoelectric effect, Compton scattering, electron pair production, or the like. It is known that, as a result, positive holes are trapped and remain at an interface or a defect, and a threshold voltage (Vth) shifts to the negative side due to the charge of this positive charge (see, for example, Japanese Unexamined Patent Application Publication No. 08-8426).
On the other hand, in the radiation imaging device of the direct conversion type as described above, the transistor is exposed to radiation, and a shift of a threshold voltage by the charge of positive holes like the one described above takes place easily. Further, even in the radiation imaging device of the indirect conversion type, some of the radiation entering the wavelength conversion layer pass through the wavelength conversion layer directly (without being converted into visible light). Therefore, there may be a case where the transistor is not a little exposed to radiation, and a shift in the threshold voltage occurs.
Thus, an attempt to reduce the shift in the threshold voltage is made, by adopting a structure in which a semiconductor layer becoming a channel is interposed between a pair of gate electrodes, the so-called dual-gate structure, thereby eliminating an influence of a back channel effect of positive holes and electrons produced in a photoelectric conversion element (see Japanese Unexamined Patent Application Publication No. 2004-265935).